Every distribution of Linux I find has its own version for everything (specially Firefox). Is there a distro with the original packages, updated when Firefox (or any other) make a release, and not when that release gets to the repositories?
@developmentalinsanity says Gentoo is updated fast, but are their sources originals or modified?
Half joking answer: http://www.linuxfromscratch.org/
Seriously, the distros don't update immediately because they perform additional testing first to ensure that the updated packages work with the rest of the system. If you want to be close to the source, Gentoo might be suitable. If you want convenience, but also staying fairly close, I'd recommend Fedora. YMMV.
Pretty much by definition, a Linux distro has its own packages, not the original ones. You seem to be asking for a distribution that isn't a distribution.
I would recommend three options, depending on how important it is for you to use the unmodified source, and your reasons for doing so.
1) LFS - short for Linux From Scratch, this un-distribution helps you build everything from original sources, making for strict adherence to your requirement. The downside is that maintenance is a formidable chore, since there is no one testing and patching for your configuration--you're the only user!
2) Gentoo - probably your best bet. You can choose your adventure, giving you the ability to do a stage 3 (installing from a bootstrapped environment, and building your system from there) , or a stage 1 (much more like LFS) but with someone doing testing and patching. The packages are darn close to mainstream.
3) Arch is also pretty close to mainstream, but slightly less so than Gentoo. Bleeding-edge sources are available with config/build packages from the community repo.
Those are really your options if you don't want to get a CS degree focused on OSes and spend your life compiling.
There's a PPA that keeps up with the latest stable releases for ubuntu which would work.
